Electrical alternans relate to the differences in electrical potential at corresponding points between alternate heartbeats. For example, Twave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or Twave, which repeats itself every beat group (e.g., two beats, three beats or four beats), and has been linked to underlying cardiac electrical instability.
Microvolt Twave alternans (TWA) has been demonstrated in many studies as a strong predictor of mortality, independent of left ventricular ejection fraction (LVEF). Macrovolt Twave alternans (TWA) has been observed prior to VT/VF (3-8 minutes) in patients with acute coronary/LQT syndrome and as such, is considered a precursor to VT/VF. Hence, detection of Twave alternans can be a potent predictor of arrhythmia onset.
Conventional Twave alternans detection methods require or assume very consistent cardiac cycle lengths (e.g., RR intervals), which inherently do not exist in intrinsic sinus rhythms. Rather, there is an intrinsic variability in cardiac cycle lengths during intrinsic sinus rhythms. Such intrinsic variability in cardiac cycle lengths make it difficult to consistently and accurately detect Twave alternans. This is because if the “same” location in Twave is not detected in every heart beat, variation, and even alternation in its amplitude may be misdetected as Twave alternans. Thus, it would be beneficial if Twave alternans, and other types of electrical alternans, could be consistently and accurately detected even when there is variability in cardiac cycle lengths.
Mechanical alternans, also known as mechanical pulse alternans (MPA), relate to the situation where alternating contractions of the heart exhibit alternating values of contraction force or magnitude that cause ejected blood to exhibit similar alternating values of diastolic pressure amplitude. For example, the presence of mechanical alternans can be defined by a consistent alternation in peak left ventricular (LV) pressure, or dP/dt, in successive beats.
The detection of mechanical alternans can be a potent predictor of congestive heart failure caused by global left ventricular dysfunction, and is considered to be a terminal sign in this population. Accordingly, it is believed that it would be useful to provide accurate methods and systems for chronically monitoring for mechanical alternans, and more generally, monitoring myocardial mechanical stability.
As with conventional Twave alternans detection methods, conventional mechanical alternans detection methods require or assume very consistent cycle lengths, which inherently do not exist in intrinsic cardiac rhythms. However, intrinsic variability in cycle lengths makes it difficult to consistently and accurately detect mechanical alternans. Accordingly, it would be beneficial if mechanical alternans could be consistently and accurately detected even when there is variability in cardiac cycle lengths.